If a grindstone is two feet in diameter, the circumference is 6.28 feet. A stone of this size has to turn at 79 revolutions a minute to have a surface speed of 500 feet a minute.
A grindstone must be run with water covering the surface of the stone to carry off the heat being produced in the tool, and to wash off any particles of steel that adhere to the surface of the stone. The disc of a grindstone is made of natural sandstone.
The grinder is a tool used for the same purpose as a grindstone but differing in the size of the wheel, the material of which it is made, and the speed at which it revolves. Grinders may be hand- or power-driven. The most common of the power-driven type are those having their wheels fastened directly to the extended shafts of a motor.
The power-driven grinder should be operated at a surface speed of from 5000 to 5500 feet a minute. The handle is attached to a large gear which engages a smaller gear. The shaft on which a smaller gear is placed extends through the casing of the grinder.
The stone is fastened to this shaft by means of a nut. The usual gear ratio of this type of grinder is 22 to 1, which means that for every revolution of the handle the shaft on which the stone is mounted revolves 22 times. If such a grinder were equipped with a stone 8" in diameter, the handle would have to be turned at about 40 r.p.m. to produce a surface speed of 5000 feet a minute.
There are three types of stones commonly used on grinders:
emery, silicon carbide, and aluminum oxide. Emery is a natural stone, whereas silicon carbide and aluminum oxide are both produced by the aid of electric furnaces. These abrasives, as the particles themselves are called, are broken up into smaller particles and are graded according to size. The small sizes are used to make stone or fine grit and the larger particles are used for coarser stones.
In manufacturing the wheels used in grinders, the abrasive grit is mixed with a bonding agent which is generally a fusible clay.
One type of whetstone — or stone used for keen-edged tools — which is used lubricated with oil is an oilstone. It is suitable for sharpening metalworking tools as well as woodworking tools. The stone may be natural, and made into a convenient shape after being taken from the earth. Or it may be fashioned from artificial abrasive materials as in the case of grindstones.
Among the more usual shapes are rectangular and wedge. The artificially made stones are of silicon carbide or aluminum oxide and may be made into wedge, rectangular, oval, round, or other special shapes, depending upon the work required. In some cases the oil is not merely placed on the stone but impregnated in it. The oil is effective in preventing the stone from getting clogged with tiny particles of the metal.
The metalworker usually has the oilstone on the top of his bench. He holds the tool to be sharpened against the stone, rubbing or honing it against the abrasive surface.
A form of oilstone which is shaped and used very much like a file is the slipstone. It is made in several cuts and many shapes, depending upon the shapes of the knives or other tools to be sharpened. It can be used on steel which would be much too hard to file. The machinist usually holds the slip-stone in his hand and rubs it over the edge he wants to sharpen or polish. Diemakers use slipstones to finish or sharpen die tools of special shapes.
When an oilstone is not being used it should be kept covered and soaked in oil, and occasionally it should be scraped off.
If no whetstone is available and a tool greatly needs whetting, a substitute can be made. Wet a flat piece of soft wood and apply a gritty substance, such as a scouring scraper. So prepared, the wood will then be a fairly good hone.
To perform properly, tools must be ground and shaped. For machine tools, this should be done on a tool grinder. A commonly used type of grinder consists of an electric motor with its shaft extended and adapted to hold grinding wheels.
Most tool grinders are equipped with two wheels, one of coarser grain than the other. The coarse-grain wheel is used for roughing down and the fine-grain wheel for finishing.
Safety laws require that guards be placed around the wheels and that transparent guards be placed so that the eyes of the user will be protected from flying particles. In one type of grinder there are simple rests upon which tools may be laid while being ground. Before the power is turned on, these rests should be tested to be sure that they are tight, and they should always be set as close to the wheel as possible.
If they are too far from the wheel, the tool being ground may slip and get caught between the rest and the wheel. This is almost certain to nick the wheel and break it. Sometimes special holders are used to hold tools in a fixed position while they are being ground.
In grinding a tool held in the hand, the tool should be kept moving across the face of the wheel to prevent grooving it. When a groove has been worn in a wheel it is necessary to dress the wheel. This is done with a wheel dresser, which has a set of star-shaped steel wheels mounted in its end. It is used by placing it on the rest and moving it across the face of the wheel while applying an even pressure.
In using the grinder, stand with your head out of line with the revolving wheel. Hand-tool grinding is only one of the many uses of grinding wheels. On specially built machines, surfaces can be ground flat and cylinders can be ground inside and out.
Hardened-steel parts which no lathe tool can cut can be readily shaped by grinding. By grinding after the piece has been hardened, any deformation due to hardening is corrected.